What is a person who is legally authorized as a public officer to witness and certify documents?

It sounds like you may be looking for a NOTARY. You must have photo ID, even if you are known to the person. Notaries are located in many places such as banks, printers, mortgage offices, etc.. Some charge a small fee. You can become a notary by contacting the secretary of state in which you reside. There are fees and education involved to do so.